Non-destructive butt weld inspection method

ABSTRACT

A non-destructive plastic butt weld inspection method in which the weld bead  42  is removed from an exterior surface of a weld zone region  44  of a butt weld and the weld zone region  44  is heated to a temperature sufficient to visualize the weld zone  44 . After heating, defective joints are readily identified by the appearance of a bond line  40  in the weld zone  44  while satisfactory joints merely show the weld zone  44  with no evidence of a bond line  40 . Surface irregularities such as ridges  46  and indentations  48  may also form in the weld zone during the heating step.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application60/260,492 filed on Jan. 9, 2001 all of which is incorporated byreference as if completely written herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to inspection of plastic joints and moreparticularly to a non-destructive plastic butt-weld joint inspectionmethod with emphasis on plastic pipe butt-weld joints.

2. Background of the Invention

Butt fusion processes have been successfully used for joining plasticpipes in water and gas-distribution systems for nearly three decades. Asin many other critical applications, the quality of the joints greatlyeffects the overall operational safety of, the system. While marfailures of pipe butt fusion joints are infrequent, when they do occur,they can be dangerous and can result in significant loss.

Typically, butt joints fall because of weakened or incomplete fusion ofthe mating surfaces. This can occur as a result of several factors. Forexample, joint failure can occur as a result of insufficient pressure atportions of or over the entire periphery of the joint. Failure can occurdue to premature solidification of the molten material in the weld zonedue to weather conditions or due to an overextended open time. That is,the molten ends of the surfaces to be joined are allowed to coolexcessively prior to the application of a bonding force. Failure canalso occur due to contamination of the molten plastic

Although ultrasonic instruments are available for nondestructiveinspection of butt fusion joints, these devices are very expensive andtheir effectiveness and reliability for field inspection has not beenfully ascertained. As a result, such instruments are not widely used.While it is possible to make cross-sectional cuts through the weld andmake visual inspection using heating techniques to bring out jointdefects, such destructive testing is useful only for training purposes.Clearly the destruction of the joint is counterproductive in a workingsystem.

Presently, there is no simple and reliable method available for thenon-destructive inspection of butt fusion joints. Typically the jointsare assessed by visual inspection based on the size and shape of theweld bead. However, this method is subjective and can be misleading. Forexample, with an excessive, joint-formation heating time, most of thepipe material is displaced into the bead during the heating cycle ratherthan as a result of the joining (forging) pressure. Although visualinspection suggests an acceptable bead appearance, the joint isdefective because of the excess loss of plastic in the joint region tothe weld. As such, visual inspection simply cannot provide directinformation as to the mechanical properties of a joint that result fromthe fusion process. The bead itself, plays an insignificant role in thejoint strength and thus is not a reliable indicator of the underlyingjoint.

To overcome these problems, it is an object of the present invention toprovide a non-destructive method of observing the quality of plasticbutt joints.

Another object of the present invention is to provide an inexpensivemethod for the field testing of plastic joints such as plastic pipejoints.

A further object of the present invention is to provide a permanentrecord of the weld joint that can be stored for future reference.

Another object of the present invention is to enhance the accuracy ofin-field weld inspection.

Yet another object of the present invention is to provide repeatableweld joint test results.

SUMMARY

To meet these and other objects of the current invention, a weld bead isremoved from an exterior surface of a weld zone area of a butt weld, theweld zone region of the butt weld is heated to a temperature sufficientto render visible bonding in a heated weld zone region. After heating,the weld zone area rendered visible by the heating is inspected. Thelack of a bond line in the weld zone region is indicative of asatisfactory joint while the presence of a bond line (an interface ofthe pieces being joined) in the weld zone region of the weld isindicative of a defective weld which should be rejected. Typically theweld bead can be removed by any appropriate means such as a cuttingtool.

The weld zone region is heated by any convenient heating means such asradiant convention, or convective heating. Radiant heat can be suppliedby an infrared lamp and convective heat by means of a hot-air gun.Heating of the weld zone region is continued to the softening point ofthe plastic that forms the butt weld.

For a permanent record of the weld, the weld zone region can bephotographed after heating is complete. If a digital camera is used toproduce a digital image, the image can be processed with imagerecognition software to detect the bond line. Alternatively laser-basedpattern recognition systems may be used for bond line detection andrecording.

The foregoing and other objects, features and advantages of theinvention will become apparent from the following disclosure in whichone or more preferred embodiments of the invention are described indetail and illustrated in the accompanying drawings. It is contemplatedthat variations in procedures, structural features and arrangement ofparts may appear to a person skilled in the art without departing fromthe scope of or sacrificing any of the advantages of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of two pipe sections placed in a axial aligningclamping device with a hot plate between the ends of the pipe sections.

FIG. 2 is a plan view of the two pipe sections shown in FIG. 1 in whichthe ends of the pipe sections have been brought in contact with the hotplate and heated sufficiently to melt the end of the pipe sections.

FIG. 3 is a plan view of the resulting butt weld of the two pipesections shown in FIG. 2 after the hot plate has been removed and theends brought in contact with each other under pressure.

FIG. 4 is a right end view of the butt weld joint shown in FIG. 3.

FIG. 5 is a cross-sectional view through the longitudinal axis of thebutt weld joint of FIG. 3 illustrating the uniform intermixing of theends of the two pipe sections in the weld zone region throughout thethickness of the pipe when the joint is formed under optimal conditions.

FIG. 6 is a plan view of the butt weld joint of FIG. 3 from which theweld bead has been removed and the weld zone region heated. Lack of theobservation of a bond line in the weld zone region demonstrates theformation of a satisfactory weld (butt joint).

FIG. 7 is a plan view of a butt weld joint weld made with insufficientinterfacial pressure in the initial joining step. The observation of thebond line and other deformities after the secondary heating step clearlyshows the formation of a defective joint.

FIG. 8 is a plan view of the weld zone region after secondary heating.Illustrating the appearance of a bond line when the ends of the piecesare overheated prior to the initial joining step. The observation of thebond line after the secondary heating step dearly shows the formation ofa defective joint.

In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology is resorted to for thesake of clarity. However, it is not intended that the invention belimited to the specific terms so selected and ft is to be understoodthat each specific term includes all technical equivalents that operatein a similar manner to accomplish a similar purpose.

Although a preferred embodiment of the invention has been hereindescribed, it is understood that various changes and modifications inthe illustrated and described structure can be affected withoutdeparture from the basic principles that underlie the invention. Changesand modifications of this type are therefore deemed to be circumscribedby the spirit and scope of the invention, except as the same may benecessary modified by the appended claims or reasonable equivalentsthereof.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE FOR CARRYING OUT THEPREFERRED EMBODIMENT

In forming a typical butt weld as shown initially in FIGS. 1-2, sections20, 22 of a thermo-plastic material such as poly-ethylene pipe areplaced in a clamping device 12 and aligned with each other as noted bythe common longitudinal center axis 50. The ends 24, 26 are planed untilthe ends are smooth and parallel. The ends 24, 26 are then brought incontact with a heated hot plate 30 (FIG. 2) until a uniform bead 32, 34becomes visible around the perimeter of both ends 24, 26. The ends 24,26 are retracted from the hot plate 30 which is removed. As seen in FIG.3, the ends 24, 26 are quickly brought together with pressure to form aweld at the interface of the ends 24, 26 of each section 20, 22,respectively. Some of the molten thermoplastic material oozes from theinterface of the ends 24, 26 of the two sections 20, 22 to form a weldbead 42. FIG. 4 is a right end view of FIG. 3 showing the weld bead 42.FIG. 5 is a cross section that illustrates the small weld bead 46 thatis formed on the interior surface of the joined pipes 20, 22 as well asthe weld zone region 44 in which the molten plastic from each section20, 22 are thoroughly intermixed and fused to each other.

As shown especially in FIGS. 3 and 6, to practice the method of thecurrent invention, the exterior weld bead 42 is removed, typically bycutting with a suitable cutting tool. At this point, the exteriorsurface of the joint is perfectly smooth and it is impossible todifferentiate a good joint from a defective joint. After the exteriorweld bead 42 has been completely removed, the weld zone region 44 isheated with a convenient heating device to a temperature sufficient torender visible the bonding between the sections 20, 22 of plasticmaterial. After this secondary heating, the weld zone region 44 isvisibly inspected and any weld exhibiting a bond line 40 is rejected.FIG. 6 illustrates a heated weld zone region 44 in which the butt-jointbetween the two sections 20, 22 was made under optimal conditions. It isto be noted that no bond line 40 between the two sections 20, 22 isvisible nor are any surface distortions present. As seen in the crosssection view of FIG. 5, the plastic from each section 20, 22 has fusedand intermingled to produce a smooth continuous region throughout theweld region that is at least as thick as the thickness of the pipe.

The secondary heating step causes a relaxation of the material in thesurface layer of the weld zone region 44 that eliminates the residualstress that developes when the melted polymer cools under pressureduring the primary welding cycle. As a result of this materialrelaxation, the outline of the weld zone 44 becomes visible. As seen inFIG. 6, for a joint formed under optimal conditions, the materialrelaxation in the bond zone 44 produced by the secondary heating ismerely a slight difference in texture over the entire bond zone 44 withno evidence of the initial interface (bond line) between the ends 24, 26of sections 20, 22 apparent.

In cases where the butt-weld is not formed under optimal conditions, theintermolecular forces between the contacting ends 24, 26 do not fullydevelop as a result of incomplete fusion and intermixing of the moltenmaterial from each end 24, 26. When a secondary heating is made,relaxation of the macromolecules at the surface causes the material topull back revealing a fusion or bond line 40.

FIG. 7 illustrates the presence of a bond line 40 between the twosections 20, 22 along with ridges 46 in the weld zone region 44 when thetwo sections 20, 22 are joined with insufficient interfacial pressure.FIG. 8 shows the presence of bond line 40 when the two sections 20, 22are joined after heating the ends of the two sections 20, 22 to too higha temperature prior to the joining operation. In addition to theappearance of the defect indicating bond line 40, ridges 46 and valleys48 may also develop on either side of the bond line 40 during theheating step. However, the observation of a bond line 40 is clearevidence of a defect weld. Butt weld failures are caused by weakened orincomplete fusion of the mating surfaces 24, 26 which give rise to thebond line 40. This can arise for a variety of reasons includinginsufficient pressure at portions or the entire interface of the joint,solidification of the material in the contact zone due to weatherconditions, an over extended open time, contamination, and otherfactors. Regardless of the cause of the weak bond. Its presence can bereadily identified by the appearance of the bond line 40 when the weldzone region 44 is rendered visible by a secondary heating step once thebond has been formed and allowed to cool.

Heating of the weld zone 44 for inspection may be carried out by anysuitable heating method including radiant, conductive or convectiveheating. Suitable heating devices for carrying out the heating stepinclude infrared lamps and hot-air guns. These devices are used to heatthe plastic of the joined sections to the softening point of the plasticforming the butt weld. At this temperature, the bond between the twosections that have been joined becomes visible and defective welds canbe easily identified by the appearance of the bond line 40 and otherbond distortions. The visible bond can be photographed for a permanentrecord of the weld. Such photography can be carried out with a digitalcamera to produce a digital image which in turn may be processed withconventional image recognition software to detect the bond line. Thevisible bond can also be analyzed and recorded with a laser-based imagerecognition system such as commercially available from Edison WeldingInstitute (Columbus Ohio).

The method is useful for thermo-plastic materials such as polyethyleneand polypropylene and is especially useful for the inspection of pipebutt welds such as those routinely found in fluid distribution systemssuch as water and gas distribution systems. The correlation betweennon-standard welding conditions and the appearance of a bond line and acorresponding reduction in joint mechanical properties was demonstratedexperimentally of which the following examples are illustrative.

EXAMPLE I Destructive Butt Weld Inspection Method—Standard WeldingConditions

Eight-inch (20.3 cm) length pipe sections 20, 22 were cut from 4-inch(10.2 cm) diameter high density polyethylene (HDPE, Phillips Driscopipe8100) pipe. Pipes were placed in the clamps of Widos Proweld 4 pipewelding unit 12 (Widos, Germany) and aligned. Pipe ends were planed witha facing unit until the ends were parallel and smooth. The facing unitwas removed and the pipe ends were brought together and checked forfinal alignment and that no visible gaps existed between the ends 24,26.

The clamps 14, 16 were retracted and a hot plate 30 at a temperature of2300° C. (4460° F.) was placed between the pipes. The pipe ends 24, 26were brought against the plate 30 firmly to ensure complete contact withan interface pressure of 0.15 MPa (21.8 psig). Completion of heating wasevident by the formation of a uniform bead 32, 34 completely around theentire circumference of both ends 24, 26 at which time the pressure wasdropped to a level sufficient to maintain the pipe ends 24, 26 incontact with the hot plate 30. At the end of the heating cycle, the ends24, 26 of the pipe 20, 22 were retracted from the hot plate 30 and thehot plate 30 was removed. The molten ends 24, 26 were then quicklybrought together and the interfacial pressure was raised to about 0.15MPa and maintained until the weld was fully cooled.

The outer weld bead 42 was removed by grinding with 120 grit sandpaper.In the field, a debidding (bead 42 removal) operation is accomplished byusing a conventional bead-cutting blade tool such as a McElroy debiddingtool (McElroy Manufacturing, Tulsa, Okla.). Removal of the weld bead 42has no significant effect on weld strength and is a technique commonlyused in many countries outside of the United States. Even in the UnitedStates, the bead 42 is removed when plastic pipe is used as a steel pipeliner and pulled into previously installed pipe.

The weld zone 44 was then made visible by carrying out a secondaryheating of the butt joint weld region 44. This heating can be done withany convenient heat source such as a radiant, convection or contact heatsource. In the present instance, the weld zone 44 was visualized bydirecting the flow of hot air from a hot air gun on the sanded surface,keeping the nozzle approximately ⅜ inch (1 cm) from the surface. Thesurface was heated until the outline of the weld zone 44 became visible.The temperature setting for the hot air gun (Leister CH 6056 Hot AirBlower type Ghibli) was 480° F.-500° F. (249° C.-260° C.). FIG. 6 showsthe exterior weld zone 44 of a joint made under optimal conditions afterthe secondary heating. The absence of the bond line or surfacedistortion shows good fusion between two sections 20, 22.

EXAMPLE II Non-Destructive Butt Weld Inspection Method—InsufficientInterfacial Pressure

The weld illustrated in FIG. 7 was prepared according to the methoddescribed above for Example I except that the interfacial pressureapplied to the two pieces of pipe during the welding process was reducedby approximately 50%. The incompleteness of the weld is evident by theappearance of a bond line 40 between the two pipe segments 20, 22 alongwith slight ridges 46 on each side of the bond line 40.

EXAMPLE III Destructive Butt Weld Inspection Method—Excessive HeatingTemperature

The weld illustrated in FIG. 8 was prepared according to the methoddescribed above for Example I except that the ends of the pipe wereoverheated during the initial welding process, that is, to a temperatureof about 2800° C. As a result, the incompleteness of the weld is clearlyevident by the appearance of the bond line 40 between the two pipesegments 20, 22 as well as ridges 46 and valleys (indentations) 48 oneither side of the bond line 40.

It is possible that changes in configurations to other than those showncould be used but that which is shown is preferred and typical. Withoutdeparting from the spirit of this invention, various plasticconfigurations other than a pipe configuration may be used as well as avariety of heating methods to render the bond line visible.

It is therefore understood that although the present invention has beenspecifically disclosed with the preferred embodiment and examples,modifications to the design concerning sizing and shape will be apparentto those skilled in the art and such modifications and variations areconsidered to be equivalent to and within the scope of the disclosedinvention and the appended claims.

1. A method of non-destructive plastic butt weld inspection comprising:a) removing a weld bead from an exterior surface of a weld zone regionof a butt weld; b) heating said weld zone region of said butt weld to atemperature sufficient to render visible bonding in said heated weldzone region; c) inspecting said weld zone region rendered visible bysaid heating; and d) rejecting said butt weld when a bond line isevident in said weld zone region.
 2. The non-destructive method ofplastic butt weld inspection according to claim 1 wherein said step ofremoving said weld bead from said weld zone region is carried out bycutting.
 3. The non-destructive method of plastic butt weld inspectionaccording to claim 1 wherein said heating step is a radiant heatingstep.
 4. The non-destructive method of plastic butt weld inspectionaccording to claim 3 wherein said radiant heating step is carried outwith an infrared lamp.
 5. The non-destructive method of plastic buttweld inspection according to claim 1 wherein said heating step is aconductive heating step.
 6. The non-destructive method of plastic buttweld inspection according to claim 1 wherein said heating step is aconvective heating step.
 7. The non-destructive method of plastic buttweld inspection according to claim 6 wherein said convective heatingstep is carried out with a hot-air gun.
 8. The non-destructive method ofplastic butt weld inspection according to claim 1 wherein said heatingstep heats said weld zone region to a softening point of said plasticforming said butt weld.
 9. The non-destructive-method of plastic buttweld inspection according to claim 1 further comprising the step ofphotographing said heated weld zone region rendered visible by saidheating step.
 10. The non-destructive method of plastic butt weldinspection according to claim 9 wherein said photographing step iscarried out with a digital camera to produce a digital image.
 11. Thenon-destructive method of plastic butt weld inspection according toclaim 10 wherein said digital image is processed with image recognitionsoftware to detect said bond line.
 12. The non-destructive method ofplastic butt weld inspection according to claim 1 further comprising thestep of detecting said bond line in said weld zone region renderedvisible by said heating step with a laser-based image recognitionsystem.
 13. The non-destructive method of plastic butt weld inspectionaccording to claim 1 wherein said butt weld is a plastic pipe butt weld.14. The non-destructive method of plastic butt weld inspection accordingto claim 13 wherein said plastic pipe butt weld is a polyethyleneplastic pipe butt weld.